1. Field of the Invention
The present invention is directed generally to digital image processing, and more particularly to estimating sensor sensitivity, e.g., from a single image. The application of the estimated sensor sensitivity to estimating scene brightness is also described.
2. Description of the Related Art
Digital images may include raster graphics, vector graphics, or a combination thereof. Raster graphics data (also referred to herein as bitmaps) may be stored and manipulated as a grid of individual picture elements called pixels. A bitmap may be characterized by its width and height in pixels and also by the number of bits per pixel. Commonly, a color bitmap defined in the RGB (red, green blue) color space may comprise between one and 16 bits per pixel for each of the red, green, and blue channels. An alpha channel may be used to store additional data such as per-pixel transparency values. Vector graphics data may be stored and manipulated as one or more geometric objects built with geometric primitives. The geometric primitives (e.g., points, lines, polygons, Bézier curves, and text characters) may be based upon mathematical equations to represent parts of digital images.
FIG. 1 illustrates the general operation of a digital camera at a high level. One or more sources of illumination, i.e., an illuminant, emit light, which bounces of objects in a scene. Some of this light enters the lens of the camera, passing the camera's aperture. The aperture is open (with fixed aperture size) for a finite amount of time, as indicated in FIG. 1. During this time, the light passing through the aperture reaches the camera's image sensor, which has a specified sensor size, as shown, where it is integrated, and converted into a digital image, depending on the sensor's sensitivity setting (ISO 12232:2006 value, referred to herein as “ISO” or “ISO value” for convenience).
Scene brightness is the amount of light illuminating a given scene. Consideration of scene brightness may allow one to guess whether a picture was taken indoors vs. outdoors because the intensity of sunlight is several orders of magnitude greater than that of any artificial light. Knowing whether a picture was taken indoors vs. outdoors, in addition with other content dependent information, can help determine the more specific environment for the photograph.
Scene brightness can be directly measured at the time a picture is taken by placing a light meter on a subject. For most nonprofessional photographs, however, such measurement is not performed. However, scene brightness is commonly calculated a posteriori from a photograph, based on several camera parameters at the time of exposure. More specifically, current scene brightness (B) computation relies on four parameters: aperture (f), exposure time (t), sensor sensitivity (S), and pixel intensity in the image (I). While it is possible to reconstruct scene brightness a posteriori, a significant portion of real life photographs are missing the required sensor sensitivity information (S). Modern digital cameras store shooting parameters in the EXIF metadata fields of a digital photograph. The parameters aperture (f) and exposure time (t) are readily available for almost all images with EXIF data. Pixel intensity (I) is stored in the pixel data and is thus always available.
Unfortunately, the same is not true for the sensor sensitivity (S) parameter. Various camera manufactures record sensor sensitivity information (commonly referred to as the ISO value) in proprietary formats and in nonstandard EXIF fields, which can be corrupted or discarded by third party image editing or transfer software. For example, only a portion of photographs in typical photo databases or archives have ISO values recorded in their metadata. Current algorithms either cannot proceed with the calculation of scene brightness altogether, or insert a constant for the ISO value in their equations, which may lead to low quality brightness estimates and may not be sufficient for many applications.